1. Field of the Invention
This invention relates to a magneto-optical recording medium for information recording and/or reproduction by a laser light or the like by taking advantage of magneto-optical effects.
2. Description of the Related Art
Recently, a magneto-optical recording system in which information is recorded by writing magnetic domains on a magnetic thin film using thermal energy of, for example, a semiconductor laser light, and is read by taking advantage of magneto-optical effects, is attracting attention as an overwritable high density recording system.
Typical of the recording material employed in this magneto-optical recording system is an amorphous alloy film combined from rare earth elements, such as Gd, Tb or Dy, and transition metals, such as Fe or Co, referred to hereinafter as an RE-TM film. This RF-TM film has many advantages. For example, it tends to become a perpendicular magnetization film indispensable for high density recording because of its larger perpendicular magnetic anisotropic energy. Also, it is amorphous and thus suffers from less noise proper to the medium, while it has a larger Kerr rotation angle. In addition, it has a lower Curie temperature so that recording and/or erasure may be achieved with a commercially available semiconductor laser with a laser power of 20 to 40 mW. As disclosed for example in JP Patent Publication KOKOKU NO. 2-32690 (1990), a GdTbFeCo film containing Tb and Gd as rare earth elements is known to have an especially large Kerr rotation angle to produce a satisfactory signal-to-noise (S/N) ratio.
On the other hand, as described for example in JP Patent Publication KOKAI No.2-56752 (1991), there is proposed a magneto-optical recording medium in which an artificial lattice film comprised of alternately stacked Co and Pt and/or Pd layers is used as a recording film. This magneto-optical recording medium, free of rare earth elements, is superior in corrosion resistance, while having a high magnetic Kerr rotation angle in a short wavelength range. Thus it is thought to be a promising recording material suitable for next-generation high-density recording making use of a short-wavelength laser.
Meanwhile, in magneto-optical recording system, it is customary to apply a bias magnetic field during recording for inverting magnetization at a region the temperature of which has been raised by laser irradiation. Therefore, in order for a recorded magnetic domain not to be affected by the bias magnetic field, it is desirable that the coercive force of the magneto-optical recording medium be sufficiently high with respect to the recording magnetic field (bias magnetic field). On the other hand, since the minimum size of the recording bit is inversely proportionate to the coercive force, a larger coercive force is similarly desired for achieving higher recording density.
In this consideration, the coercive force of the magneto-optical recording medium employing the above mentioned artificial lattice film as a recording film cannot be said to be completely satisfactory.